This invention relates to an improved apparatus for deriving spectral information and quantifying the physical properties of a sample. More particularly, this invention relates to an improved near infrared spectrophotometer apparatus, particularly suited for determining the physical properties of a sample in an industrial environment.
The physical properties of sample materials which, for purposes of the present invention encompass the physical, chemical, and fuel properties of sample materials, have historically been measured one property at a time, utilizing test methods which have been developed to specifically evaluate one particular property. For example, the heat of formation of a particular sample has been determined by actually burning the sample in a calorimeter. Similarly, the molecular weight of a sample has been determined by inducing and measuring viscous flow of the sample using a viscometer. In each of these examples, however, the physical test methods measure, or quantify, the physical properties by actually subjecting the sample to the conditions in question. To measure more than one physical property of a particular sample, a plurality of tests must be individually conducted on a plurality of samples. Often these samples are destroyed or consumed in the process. These approaches to measuring physical properties are slow, expensive, subject to testing inconsistency, and do not facilitate on-line or real time use in an industrial or field setting.
More recently, spectrophotometric analysis has been used to determine indirectly the quantitative properties of sample materials.
U.S. Pat. No. 4,800,279 to Hieftje et al. discloses a method for utilizing near-infrared absorbance spectra to identify the physical properties of gaseous, liquid, or solid samples. The method requires measuring and recording the near-infrared absorbance spectra of a representative set of calibration samples and employing a row-reduction algorithm to determine which wavelengths in the near-infrared spectrum are statistically correlated to the physical property being quantified and to calculate weighting constants which relate the absorbance at each wavelength to the physical property being monitored. The near-infrared absorbance of a sample can then be measured at each of the correlated wavelengths and multiplied by the corresponding weighting constant. The physical property being quantified is then computed from the sum of the products of the absorbance of the sample and the corresponding weighting constant at the correlated wavelengths.
Use of spectrophotometric analysis has numerous advantages over other methods since it is rapid, relatively inexpensive, and multivariate in that many wavelengths can be measured and therefore many properties can be monitored simultaneously. While the potential for spectrophotometric analysis in manufacturing facilities, chemical plants, petroleum refineries, and the like is great, several obstacles must be overcome in order to achieve successful implementation from a practical viewpoint. These obstacles include development of an apparatus that is field durable, accurate, and stable over time under generally adverse conditions.
Most spectrophotometers typically include a light source, a grating for dispersing light in a series of monochromatic, single wavelength beams, and a suitable photodetector. The grating may be positioned to provide predispersed monochromatic light to both the sample and then the detector or, alternatively, polychromatic light from the source may be directed onto the sample and then post-dispersed by the grating before being directed to the detector. Post-dispersion permits analysis of several wavelengths simultaneously.
Several U.S. Patents have illustrated the problems and progress made towards development of a field rugged, accurate, and stable spectrophotometric device, each meeting with varying degrees of success.
One method of improving the photometric precision of prior art spectrophotometers was to provide a stable light source for illuminating the sample under analysis. For example, U.S. Pat. No. 4,094,609 teaches a means for enhancing the consistency and uniformity of the light output from the source used to irradiate the sample. However, even the best methods of providing a stable light source generally yield discernible variations in light intensity at the wavelengths of interest.
A subsequent improvement in photometric precision was made with the addition of a reference spectral pattern which could be used analytically to account for variations in the light intensity of the sample spectral pattern which were not attributable to light interaction with the sample. Spectrophotometers having a sample and reference channel are referred to as dual-channel spectrophotometers. U.S. Pat. Nos. 4,820,045 to Boisde et al., 4,932,779 to Keane, and 4,755,054 to Ferree teach use of fiber optic bundles, having multiple strands of fiber optic cable with one or more strands dedicated exclusively for providing a reference spectrum. It was subsequently found, however, that each particular fiber in the fiber optic bundle, sampled a different location on the filament of the light source and launched their respective transmitted light to different locations in the spectrophotometer. Small differences in the launching of light into and out of the fiber optics created discernible artifacts in the measured sample and reference spectra. Minor variations caused by filament vibration, small differences in intensity and color temperature along the length of the filament, inhomogeneity in the optics, and other phenomena induce different changes in the sample and reference spectra, which introduced substantial errors in quantifying the spectra. Moreover, spectrophotometric devices having fiber optic bundles generally need to be recalibrated and the chemometric model rebuilt for even the most trivial of maintenance tasks such as the routine replacement of the light source. This model rebuilding step can require the collection and the analysis of from 20 to 100 representative samples prior to proceeding with chemometric model rebuilding. These activities are costly and time consuming. Furthermore, fiber optic bundles are also substantially more costly than single fiber optic strands.
Many of the inherent problems with fiber optic bundles were addressed with the use of single strand fiber optic cable and means to launch the light alternatively to and from the sample and reference channels through the same fiber optic strand. The various means for alternatively directing light to the sample and reference channels are described in several U.S. patents.
U.S. Pat. No. 4,938,555 to Savage teaches the use of a single fiber optic strand having a moving mirror-type fiber optic light diverting means for directing light from a single fiber to one of a plurality of selected locations. Moving mirror-type fiber optic light-diverting devices generally present a number of obstacles to constructing and utilizing such a device, particularly in a manufacturing environment. The moving-mirror fiber optic diverting devices require that the light launching fibers be precisely aligned with each receiving fiber. This is particularly difficult and requires several critical multidimensional alignments of the optical components. These critical alignments are also particularly vulnerable to vibrationally-induced misalignment, a common concern in a field or manufacturing environment. These critical alignments are also subject to wear in the mechanical devices used to drive the mirror and select among the multiple ports of the diverting mechanism.
Moreover, the efficiency of transmitting light power from the light launching to receiving fibers is inherently low in such light-diverting devices As described in U.S. Pat. No. 4,820,045 to Boisde et al., "apart from the transmission loss due to the actual fibre, there are certain light energy loss causes at the junctions of the fibres, (particularly in collimating lenses and also during reflections at the intake of the fibres)."
Another fiber optic light-diverting means employs the use of a chopper or shutter to selectively launch light from a single fiber to a plurality of receiving ports. An example of such a light-diverting means is disclosed in U.S. Pat. No. 4,755,054 to Ferree wherein a rotatable chopper means is used to direct light from a plurality of light sources to a receiving fiber. While the chopper and shutter devices reduce the need for a critical moving optical component, chopper and shutter devices transmit light power from the light launching to receiving fibers at a particularly low efficiency. The light efficiencies inherent to these devices can be, and are generally lower than 50% of the light power introduced into the launching fiber.
Another limitation attendant to many previously disclosed spectrophotometric devices is in the means used to resolve and measure light at the various and particular wavelengths. For example, in international Application published under the Patent Cooperation Treaty WO 90/07697 to Lefebre, wavelength resolution is achieved using a moving grating monochromator. The moving grating monochromator comprises a diffraction grating rotating about a central axis relative to the light source for alternately projecting light from each narrow wavelength band onto the fiber optics leading to the sample and reference channels and subsequently to the detector. The precision and reliability of the device is limited by the repeatability of the mechanical motion of the diffraction grating. Wear on the mechanical components used to produce the motion, such as bearings, ultimately limits the wavelength precision of spectrophotometers of this design and eventually necessitates recalibration of the analyzer and development of a new chemometric model.
Alternative monochromator means have been developed to correct some of the deficiencies associated with monochromators having a moving diffraction grating. For example, some previous spectrophotometers have employed various types of filters rather than diffraction gratings for wavelength resolution. U.S. Pat. No. 4,883,963 to Kemeny et al., teaches the use of an acousto-optical tunable filter (AOTF) for single wavelength resolution. While an AOTF obviates the need for a critical mechanical motion device in the monochromator, these devices share a common limitation with moving diffraction grating monochromators in that each wavelength in the spectrum must be analyzed sequentially and cannot be analyzed simultaneously. This limits the speed with which the spectrum can be measured and increases the cycle time for providing determinations from the analyzer. Moreover, the sequential scanning of the wavelengths can introduce artifacts and irreproducibilities, particularly in a field environment when, for example, gas bubbles appear in the sample or the sample composition changes abruptly.
We have now found that many of the previously disclosed fiber optic bundle and light diverting devices introduce artifacts into the spectrum which degrade the accuracy and precision of the analysis. These artifacts can occur from the imprecise or irreproducible imaging of light from the launching into the receiving fiber optics and generally appear as irreproducibilities in the measured spectrum. Relatively small misalignments in the imaging can result in significant irreproducibilities in the measured spectrum, with these irreproducibilities being particularly significant when the spectrophotometer is used to determine the physical properties of samples. While these irreproducibilities may be less likely to occur in a controlled laboratory environment, they are assured in an industrial facility where on-line analysis can place an analyzer under particularly harsh conditions.
We have found that the use of a mode scrambler, particularly in spectrophotometers using single fiber optic strands for the transmission of light to and from the sample, mitigates many of the effects caused by irreproducible imaging and small misalignments, which significantly improves the precision of spectral measurements. These irreproducibilities and misalignments cause unpredictable and non-uniform changes in the angular distribution of light launched from the fiber optic strand into the wavelength resolving device of the spectrophotometer. By scrambling or redistributing the modes of light propagation in the fiber optic strand, the mode scrambler gives a uniform and reproducible image of the light from the fiber optic strand into the wavelength resolving device of the spectrophotometer, which is essential for obtaining precise spectral measurements.
We have also found that the use of high efficiency fiber optic switches in tandem with a mode scrambling device to reproducibly and uniformly image light into the spectrophotometer, provides a substantial improvement in chemometric prediction precision. It has similarly been found that the addition of a spectrophotometer having a fixed diffraction grating, a single fiber optic strand for launching light from the sample and reference channels into the spectrophotometer, and an array of photodetectors for measuring the light intensity at multiple wavelength simultaneously, provides a substantial and further improvement in chemometric prediction precision, speed of analysis, and instrument versatility.
It is therefore an object of the present invention to provide an apparatus for chemometric prediction with superior chemometric prediction accuracy, reliability, durability, and stability over time, suitable for use in a manufacturing or field environment.
It is another object of the present invention to provide an apparatus for chemometric prediction without the inherent cost and inaccuracies of fiber optic bundles.
It is another object of the present invention to provide an apparatus for chemometric prediction that efficiently and precisely measures the light transmitted through the sample and reference channels of the analyzer with desensitized fiber optic switches and without other diverting devices that rely on the precise mechanical alignment of a critical optical component.
It is another object of the present invention to provide an apparatus for chemometric prediction that precisely, reproducibly, and expeditiously measures absorbances at all relevant wavelengths and is not limited to sequential wavelength measurement.
Other objects appear herein.